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1. H 2 O 2 and CH 3 OOH (MHP) in the Remote Atmosphere: 2. Physical and Chemical Controls

   https://doi.org/10.1029/2021JD035702  

   Allen, Hannah M. ; Bates, Kelvin H. ; Crounse, John D. ; Kim, Michelle J. ; Teng, Alexander P. ; Ray, Eric A. ; Wennberg, Paul O. ( March 2022 , Journal of Geophysical Research: Atmospheres)

   Hydrogen peroxide (H₂O₂) and methyl hydroperoxide (MHP, CH₃OOH) serve as HO_x(OH and HO₂radicals) reservoirs and therefore as useful tracers of HO_xchemistry. Both hydroperoxides were measured during the 2016–2018 Atmospheric Tomography Mission as part of a global survey of the remote troposphere over the Pacific and Atlantic Ocean basins conducted using the NASA DC‐8 aircraft. To assess the relative contributions of chemical and physical processes to the global hydroperoxide budget and their impact on atmospheric oxidation potential, we compare the observations with two models, a diurnal steady‐state photochemical box model and the global chemical transport model Goddard Earth Observing System (GEOS)‐Chem. We find that the models systematically under‐predict H₂O₂by 5%–20% and over‐predict MHP by 40%–50% relative to measurements. In the marine boundary layer, over‐predictions of H₂O₂in a photochemical box model are used to estimate H₂O₂boundary‐layer mean deposition velocities of 1.0–1.32 cm s⁻¹, depending on season; this process contributes to up to 5%–10% of HO_xloss in this region. In the upper troposphere and lower stratosphere, MHP is under‐predicted and H₂O₂is over‐predicted by a factor of 2–3 on average. The differences between the observations and predictions are associated with recent convection: MHP is under‐estimated and H₂O₂is over‐estimated in air parcels that have experienced recent convective influence.

    

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2. A coastal N2 fixation hotspot at the Cape Hatteras front: Elucidating spatial heterogeneity in diazotroph activity via supervised machine learning

   https://doi.org/10.1002/lno.11727  

   Selden, Corday R. ; Chappell, P. Dreux ; Clayton, Sophie ; Macías‐Tapia, Alfonso ; Bernhardt, Peter W. ; Mulholland, Margaret R. ( April 2021 , Limnology and Oceanography)

   In the North Atlantic Ocean, dinitrogen (N₂) fixation on the western continental shelf represents a significant fraction of basin‐wide nitrogen (N) inputs. However, the factors regulating coastal N₂fixation remain poorly understood, in part due to sharp physico‐chemical gradients and dynamic water mass interactions that are difficult to constrain via traditional oceanographic approaches. This study sought to characterize the spatial heterogeneity of N₂fixation on the western North Atlantic shelf, at the confluence of Mid‐ and South Atlantic Bight shelf waters and the Gulf Stream, in August 2016. Rates were quantified using the¹⁵N₂bubble release method and used to build empirical models of regional N₂fixation via a random forest machine learning approach. N₂fixation rates were then predicted from high‐resolution CTD and satellite data to infer the variability of its depth and surface distributions, respectively. Our findings suggest that the frontal mixing zone created conditions conducive to exceptionally high N₂fixation rates (> 100 nmol N L⁻¹d⁻¹), which were likely driven by the haptophyte‐symbiont UCYN‐A. Above and below this hotspot, N₂fixation rates were highest on the shelf due to the high particulate N concentrations there. Conversely, specific N₂uptake rates, a biomass‐independent metric for diazotroph activity, were enhanced in the oligotrophic slope waters. Broadly, these observations suggest that N₂fixation is favored offshore but occurs continuously across the shelf. Nevertheless, our model results indicate that there is a niche for diazotrophs along the coastline as phytoplankton populations begin to decline, likely due to exhaustion of coastal nutrients.

    

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3. Biomass Burning Markers and Residential Burning in the WINTER Aircraft Campaign

   https://doi.org/10.1029/2017JD028153  

   Sullivan, A. P. ; Guo, H. ; Schroder, J. C. ; Campuzano‐Jost, P. ; Jimenez, J. L. ; Campos, T. ; Shah, V. ; Jaeglé, L. ; Lee, B. H. ; Lopez‐Hilfiker, F. D. ; et al ( February 2019 , Journal of Geophysical Research: Atmospheres)

   As part of the WINTER (Wintertime Investigation of Transport, Emissions, and Reactivity) campaign, a Particle‐into‐Liquid Sampler with a fraction collector was flown aboard the National Center for Atmospheric Research C‐130 aircraft. Two‐minute integrated liquid samples containing dissolved fine particulate matter (PM₁) species were collected and analyzed off‐line for the smoke marker levoglucosan using high‐performance anion‐exchange chromatography‐pulsed amperometric detection to compare levoglucosan with aerosol mass spectrometer (AMS) biomass burning markers and investigate the contribution from residential burning during the study. Levoglucosan was correlated with AMS organic aerosol (R² = 0.49) and with carbon monoxide (CO;R² = 0.51) for all flights. Levoglucosan was not correlated with the inorganic smoke marker water‐soluble potassium but was correlated with the AMS markers ∆C₂H₄O₂⁺(high resolution,R² = 0.60) and ∆m/z60 (unit mass resolution,R² = 0.61). However, at low levoglucosan, AMS markers deviated potentially due to interferences from other sources or differences with the species captured by the AMS markers. Analysis of levoglucosan changes relative to carbon monoxide as plumes advected from source regions showed no systematic levoglucosan loss for plumes up to 20 hr old. Based on literature residential burning source ratios and measured levoglucosan, contributions of organic carbon (OC) due to residential burning were estimated. The contribution ranged from ~30 to 100% of the OC, with significant variability depending on the source ratio used; however, the results show that biomass burning was a significant PM₁OC source across the entire sampling region. A GEOS‐Chem model simulation predicted significantly less smoke contribution.

    

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4. Gallium: A New Tracer of Pacific Water in the Arctic Ocean

   https://doi.org/10.1029/2019JC015842  

   Whitmore, Laura M. ; Pasqualini, Angelica ; Newton, Robert ; Shiller, Alan M. ( June 2020 , Journal of Geophysical Research: Oceans)

   Determining the proportions of Atlantic and Pacific Ocean seawater entering the Arctic Ocean is important both for understanding the mass balance of this basin as well as its contribution to formation of North Atlantic deep water. To quantify the distribution and amount of Pacific and Atlantic origin seawater in the western Arctic Ocean, we used dissolved Ga in a four‐component linear endmember mixing model. Previously, nutrients, combined in their Redfield ratios, have been used to separate Pacific‐ and Atlantic‐derived waters. These nutrient tracers are not conservative in practice, and there is a need to find quantities that are conserved. Dissolved Ga concentrations show measurable contrast between Atlantic and Pacific source waters, shelf‐influenced waters show little impact of shelf processes on the dissolved Ga distribution, and dissolved Ga in the Arctic basins is conserved along isopycnal surfaces. Thus, we explored the potential of Ga as a new parameter in Arctic source water deconvolution. The Ga‐informed deconvolution was compared to that generated with the NO₃:PO₄relationship. While distributions of the water masses were qualitatively similar, the Ga‐based deconvolution predicted higher amounts of Pacific water at depths between 150 and 300 m. The Ga‐based decomposition yields a smoother transition between the halocline and Atlantic layers, while nutrient‐based solutions have sharper transitions. A 1‐D advection‐diffusion model was used to constrain estimates of vertical diffusivity (K_z). The Ga‐based K_zestimates agreed better with those from salinity and temperature than the nutrient method. The Ga‐based approach implies greater vertical mixing between the Pacific and Atlantic waters.

    

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5. Glacial Ice Sheet Extent Effects on Modeled Tidal Mixing and the Global Overturning Circulation

   https://doi.org/10.1029/2019PA003644  

   Wilmes, S. ‐B. ; Schmittner, A. ; Green, J. A. M. ( August 2019 , Paleoceanography and Paleoclimatology)

   At present, tides supply approximately half (1 TW) of the energy necessary to sustain the global deep meridional overturning circulation (MOC) through diapycnal mixing. During the Last Glacial Maximum (LGM; 19,000–26,500 years BP), tidal dissipation in the open ocean may have strongly increased due to the 120‐ to 130‐m global mean sea level drop and changes in ocean basin shape. However, few investigations into LGM climate and ocean circulation consider LGM tidal mixing changes. Here, using an intermediate complexity climate model, we present a detailed investigation on how changes in tidal dissipation would affect the global MOC. Present‐day and LGM tidal constituents M₂, S₂, K₁, and O₁are simulated using a tide model and accounting for LGM bathymetric changes. The tide model results suggest that the LGM energy supply to the internal wave field was 1.8–3 times larger than at present and highly sensitive to Antarctic and Laurentide ice sheet extent. Including realistic LGM tide forcing in the LGM climate simulations leads to large increases in Atlantic diapycnal diffusivities and strengthens (by 14–64% at 32°S) and deepens the Atlantic MOC. Increased input of tidal energy leads to a greater drawdown of North Atlantic Deep Water and mixing with Antarctic Bottom Water altering Atlantic temperature and salinity distributions. Our results imply that changes in tidal dissipation need be accounted for in paleoclimate simulation setup as they can lead to large differences in ocean mixing, the global MOC, and presumably also ocean carbon and other biogeochemical cycles.

    

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